1. Field of the Invention
The present invention relates to a UHF transistor mixer circuit. More specifically, the present invention relates to a UHF transistor mixer circuit which includes a transistor, a bias voltage from a direct current voltage source being supplied to the base of the transistor through an inductor constructed with a microstripline, and an inputted high-frequency signal and a local oscillation signal being inputted to the base.
1. Description of the Prior Art
A conventional mixer circuit is shown in FIG. 2. With reference to FIG. 2, a high-frequency signal and a local oscillation signal are inputted through an input terminal IN to a mixer circuit. The high-frequency signal is outputted by a high-frequency amplifying circuit (not shown) and has a frequency of 810-835 MHz, for example. The local oscillation signal is outputted by a local oscillating circuit (not shown) and has a frequency of 720-745 MHz, for example. The two signals are inputted to a base of a transistor TR via the input terminal IN. A voltage obtained by voltage-dividing a direct current voltage +B by means of resistors R1 and R2 is applied as a bias voltage to the base of the transistor TR via an inductor L1 constructed by a microstripline. The inductor L1 becomes a high-impedance with respect to the signals to be inputted to the base of the transistor TR, that is, the high-frequency signal, and the local oscillation signal so as to prevent the respective inputted signals from flowing toward the direct current voltage source +B.
An emitter resistor R3 and a bypass capacitor C1 connected in parallel between each other are connected to an emitter of the transistor TR and ground. A collector of the transistor TR is connected to an output terminal OUT of the mixer circuit, and the output terminal OUT, i.e. the collector of the transistor TR, is connected to the direct current voltage source +B via a choke coil L2.
In the mixer circuit shown in FIG. 2, an intermediate frequency signal having a frequency equal to the difference between the inputted high-frequency signal and the local oscillation signal is outputted from the collector of the transistor TR, i.e. the output terminal OUT.
One of the important elements which determine the performance of such a UHF transistor mixer circuit is the intermodulation distortion, and it is desired that the intermodulation distortion be as small as possible. In order to make the intermodulation distortion small, it is necessary to prevent the local oscillation signal from flowing toward the direct current voltage source +B in a case where the injected level of the local oscillation signal to the base of the transistor TR is constant. However, in the conventional UHF transistor mixer circuit shown in FIG. 2, even though the inductor L1 composed of a microstripline is inserted, the degree of improvement of the intermodulation distortion is insufficient, because the local oscillation signal flows toward the direct current voltage source +B through the inductor L1.